New generation power supplies are subject to very fast changing load currents, which are difficult to limit. As a result, there is a requirement for current limiting circuits to incorporate appropriate circuits that sense the current flow through a transistor or other circuit, limit the amount of voltage or current, and permit discharge. These types of circuits are used in hot plug circuits, linear regulators, power converters, and similar circuits. Also, new generation power supplies are subject to very fast changing load currents, which are difficult to limit with existing circuit designs.
Prior art current limiting circuits typically have used a current regulation feedback loop containing a large output device operative as a switch. For example, in one type of current limiting circuit, an external Metal Oxide Semiconductor Field Effect Transistor (MOSFET) has its gate connected to a single output of a transconductance amplifier, which is operative as an error amplifier. The prior art current regulation feedback circuit includes the error amplifier, an external output transistor, and a current sensing resistor. These components connect to the drain forming a feedback loop. The current sensing resistor is typically connected to a set resistor to aid in providing a reference, and in turn, connects to an inverting input INV of the transconductance amplifier. The non-inverting input is typically connected between the output transistor, for example, the external MOSFET, and the current sensing resistor.
In this conventional type of circuit, the feedback loop is normally slow because of the large capacitive loading on the gate of the output transistor, typically as the output of a sense amplifier. The over-current situation in this circuit is not detected until the gate of this large output transistor is pulled down to a level where the feedback loop goes into regulation. The over-current is sensed and the information is available at the output of the error amplifier. Because of the large gate capacitance, the loop slows and information is not transferred back to the input of the error amplifier fast enough to accomplish regulation.
It would be advantageous to solve this prior art drawback of fast current limiting in these prior art devices and circuits as described above. Some prior art circuits have used high speed operational amplifiers, for example, as commonly found in some hot plugs to form a quick response circuit output. They are also often used in prior art linear regulators. These circuits, however, can be expensive and complicated to implement.